Light emitting device for AC operation

ABSTRACT

An AC light emitting device is disclosed. The AC light emitting device includes at least four substrates. Serial arrays each of which has a plurality of light emitting cells connected in series are positioned on the substrates, respectively. Meanwhile, first connector means electrically connect the serial arrays formed on respective different substrates. At least two array groups each of which has at least two of the serial arrays connected in series by the first connector means are formed. The at least two array groups are connected in reverse parallel to operate. Accordingly, there is provided an AC light emitting device capable of being driven under an AC power source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/KR2007/004268, filed Sep. 5, 2007, and claims priority from and thebenefit of Korean Patent Application No. 10-2006-0114553, filed on Nov.20, 2006, which are both hereby incorporated by reference for allpurposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device, and moreparticularly, to an AC light emitting device, which can be driven bybeing connected directly to an AC power source.

2. Discussion of the Background

With the development of GaN based light emitting diodes (LEDs), the GaNbased LEDs have considerably changed LED technologies. Currently, theGaN based LEDs are used for various applications such as full-color LEDdisplays, LED traffic lights, white LEDs and the like. Recently, it hasbeen expected that high-efficiency white LEDs will substitute forfluorescent lamps. In particular, the efficiency of white LEDs hasreached the level similar to that of typical fluorescent lamps.

In general, an LED emits light by forward current and requires thesupply of DC. Hence, if the LED is connected directly to an AC powersource, it is repeatedly turned on/off depending on the direction ofcurrent. As a result, there are problems in that the LED does notcontinuously emit light and is easily broken by reverse current.

To solve such a problem, an LED capable of being connected directly to ahigh-voltage AC power source is disclosed in PCT Patent Publication No.WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVINGLIGHT-EMITTING ELEMENTS” by SAKAI et al.

According to PCT Patent Publication No. WO 2004/023568(A1), LEDs (i.e.,light emitting cells) are two-dimensionally connected in series on asingle insulative substrate such as a sapphire substrate to form LEDarrays. Such two LED arrays are connected to each other in reverseparallel on the sapphire substrate. As a result, there is provided asingle chip light emitting device capable of being directly driven by anAC power supply.

However, in the single chip light emitting device, a failure of any oneof the light emitting cells connected in series or disconnection/shortcircuit of wires results in a chip failure that makes the AC operationof the device impossible. In particular, in a cast that the single chipincludes a few tens of light emitting cells and wires for connectingthem in series and reverse parallel so as to be driven under a highvoltage, e.g., a 110/220V AC power source used for general household, achip failure may be easily occurred due to a large number of the lightemitting cells and the wires formed on the single chip.

Further, as a plurality of light emitting cells are connected in seriesand reverse parallel on the same substrate, it is likely that shortcircuits between wires for connecting the light emitting cells increase,and the processes of patterning the light emitting cells arecomplicated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an AC light emittingdevice, which can appropriately reduce the number of light emittingcells and wires formed on a single substrate.

Another object of the present invention is to provide an AC lightemitting device capable of simplifying patterning and wire connectingprocesses of light emitting cells formed on a single substrate.

A further object of the present invention is to provide an AC lightemitting device capable of preventing overvoltage from being applied tolight emitting cells in an array to which reverse voltage is applied byan AC power source.

According to one aspect of the present invention for achieving theobjects, an AC light emitting device includes at least four substrates.Serial arrays each of which has a plurality of light emitting cellsconnected in series are positioned on the substrates, respectively.Meanwhile, first connector means electrically connect the serial arraysformed on respective different substrates. At least two array groupseach of which has at least two of the serial arrays connected in seriesby the first connector means are formed. The at least two array groupsare connected in reverse parallel to operate. Accordingly, it ispossible to provide an AC light emitting device capable of reducing thenumber of light emitting cells formed on a single substrate. Further,since all the light emitting cells formed on the single substrate can beconnected in series, the patterning and wire forming processes of thelight emitting cells can be simplified.

Meanwhile, at least one second connector means may electrically connectthe first connector means corresponding to each other provided in thearray groups connected in reverse parallel. The second connector meansprevent overvoltage from being applied to a specific array in the arraygroup to which reverse voltage is applied during operation. Accordingly,it is possible to protect the light emitting cells in the array group towhich a reverse voltage is applied.

Meanwhile, bonding pads may be positioned on the respective substrates.The bonding pads are electrically connected to both ends of each of theserial arrays, and the first connector means connect the bonding pads tothereby connect the serial arrays in series.

The at least four substrates may be mounted in a single package. At thistime, the first connector means may be bonding wires for directlyconnecting the bonding pads.

Meanwhile, the at least four substrates may be mounted in differentpackages, respectively. At this time, the bonding pads on the substrateare electrically connected to lead electrodes in a package, and the leadelectrodes of such packages are electrically connected to one another,thereby forming array groups connected in series. Here, the bonding padsmay be connected to the lead electrodes in the package in variousmanners. For example, the bonding pads may be connected through bondingwires.

The number of substrates mounted in one package may vary. For example,one substrate is mounted in one package, and at least four or more ofsuch packages are connected to one another to thereby form an AC lightemitting device. Further, at least two or more substrates are mounted ineach package and connected in series to form an array group and suchpackages are connected to one another, whereby an AC light emittingdevice can be configured. Furthermore, at least two or more substratesare mounted in each package and connected in reverse parallel and suchpackages are connected to one another, whereby an AC light emittingdevice having array groups with the serial arrays connected in seriescan be configured.

According to another aspect of the present invention for achieving theobjects, an AC light emitting device includes at least two substrates.First and second serial arrays connected in reverse parallel arepositioned on each of the substrates. The serial arrays are formed byconnecting a plurality of light emitting cells in series to one another.Meanwhile, first connector means electrically connect serial arrays toeach other on the respective different substrates to thereby form atleast two array groups. Further, second connector means are formed onthe substrates, respectively, to electrically connect the light emittingcells corresponding to each other provided in the first and secondarrays connected in reverse parallel.

According to embodiments of the present invention, a plurality of singlechips having an serial array of light emitting cells are used, wherebythe number of the light emitting cells and wires formed on a singlesubstrate can be appropriately reduced. Accordingly, a chip failure ratein a manufacturing process can be decreased, thereby reducingmanufacturing costs. Further, since it is not required to connect lightemitting cells formed on a single substrate in reverse parallel, thepatterning and wire connecting process of the light emitting cellsformed on the single substrate can be simplified. Furthermore, secondconnector means for connecting array groups to one another are employed,thereby providing an AC light emitting device capable of preventingovervoltage from being applied to light emitting cells in an array towhich a reverse voltage is applied. In addition, since a plurality ofsingle chips are used, the single chips for emitting lights withdifferent light emitting wavelengths can be disposed, thereby providingan AC light emitting device for emitting lights with variouswavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a serial array of light emittingcells according to embodiments of the present invention.

FIGS. 2 and 3 are partial sectional views illustrating the lightemitting cells used in the embodiments of the present invention.

FIG. 4 is a schematic view of an AC light emitting device according toan embodiment of the present invention.

FIG. 5 is a schematic view of an AC light emitting device according toanother embodiment of the present invention.

FIG. 6 is a sectional view illustrating a package used in theembodiments of the present invention.

FIG. 7 is a schematic view illustrating serial arrays of light emittingcells according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are provided only for illustrative purposes sothat those skilled in the art can fully understand the spirit of thepresent invention. Therefore, the present invention is not limited tothe following embodiments but may be implemented in other forms. In thedrawings, the widths, lengths, thicknesses and the like of elements maybe exaggerated for convenience of illustration. Like reference numeralsindicate like elements throughout the specification and drawings.

FIG. 1 is a schematic view illustrating a serial array of light emittingcells according to embodiments of the present invention. Here, theserial arrays are disposed in a single chip 50.

Referring to FIG. 1, the single chip 50 includes a substrate 51. Thesubstrate 51 may be an insulative substrate or a conductive substratehaving an insulating layer on a top surface thereof. A plurality oflight emitting cells 58 are disposed on the substrate 51. The lightemitting cells are serially connected to one another through wires toform a serial array 61. Bonding pads 71 may be disposed at both ends ofthe serial array 61. The bonding pads 71 are electrically connected toboth ends of the serial array 61, respectively.

In the embodiments of the present invention, all the light emittingcells in the single chip 50 may be connected in series on a singlesubstrate. Hence, the processes of forming the light emitting cells 58on a single substrate and forming wires for connecting the lightemitting cells 58 are more simplified as compared with a related art.

FIGS. 2 and 3 are partial sectional views illustrating the lightemitting cells. Here, FIG. 2 is a partial sectional view illustratingthe light emitting cells connected in series through wires formedthrough an air bridge process, and FIG. 3 is a partial sectional viewillustrating the light emitting cells connected in series through wiresformed through a step cover process.

Referring to FIG. 2, a plurality of light emitting cells 58 arepositioned on a substrate 51 to be spaced apart from one another. Eachof the light emitting cells 58 comprises a first conductive-type lowersemiconductor layer 55, an active layer 57 and a second conductive-typeupper semiconductor layer 59. The active layer 57 may be formed in asingle or multiple quantum well structure, and the material andcomposition of the active layer may be selected depending on a requiredlight emitting wavelength. For example, the active layer may be formedof a GaN-based compound semiconductor. Meanwhile, the lower and uppersemiconductor layers 55 and 59 may be formed of a material with abandgap larger than the active layer 57, and may be formed of aGaN-based compound semiconductor.

Meanwhile, a buffer layer 53 may be interposed between the lowersubstrate 55 and the substrate 51. The buffer layer 53 is employed toreduce lattice mismatch between the substrate 51 and the lowersemiconductor layer 55. Although the buffer layers 53 may be spacedapart from one another as shown in this figure, the present invention isnot limited thereto. That is, when the buffer layers 53 are formed of aninsulative material or a material with large resistance, they may becontinuously formed.

As shown in this figure, the upper semiconductor layer 59 is positionedon a region of the lower semiconductor layer 55, and the active layer 57is interposed between the upper and lower semiconductor layers 59 and55. Further, a transparent electrode layer 61 may be positioned on theupper semiconductor layer 59. The transparent electrode layer 61 may beformed of a material including indium tin oxide (ITO), Ni/Au, or thelike.

Meanwhile, wires 67 electrically connect the light emitting cells 58 toone another. Each of the wires 67 connects the lower semiconductor layer55 of one of the light emitting cells to the transparent electrode layer61 of another of the light emitting cells adjacent thereto. As shown inthis figure, the wires may connect an electrode pad 64 formed on thetransparent electrode layer 61 and an electrode pad 65 formed on theexposed region of the lower semiconductor layer 55. Here, the wires 67are formed through an air bridge process. Accordingly, the wires 67except contacts are physically separated from the substrate and thelight emitting cells 58. The serial array 61 (in FIG. 1) having thelight emitting cells connected in series on the single substrate 51through the wires 67 is formed.

Referring to FIG. 3, wires for connecting the light emitting cells 58can be formed through a step cover process. That is, all the layers ofthe light emitting cells and the substrate 51 except portions to whichthe wires 87 are connected are covered with an insulating layer 85. Inaddition, the wires 87 are patterned on the insulating layer 85 toelectrically connect the light emitting cells 58 to one another.

For example, the insulating layer 85 has openings for exposing theelectrode pads 64 and 65. The wires 87 connect the electrode pads 64 and65 of the adjacent light emitting cells to each other through theopenings, thereby connecting the light emitting cells in series.

FIG. 4 is a schematic view of an AC light emitting device according toan embodiment of the present invention.

Referring to FIG. 4, the AC light emitting device comprises four singlechips 50 a, 50 b, 50 c and 50 d. Each of the single chips 50 a, 50 b, 50c and 50 d has the same components as the single chip 50 described withreference to FIG. 1. For convenience, a, b, c and d are added toreference numerals of the respective components.

Light emitting cells 58 a, 58 b, 58 c or 58 d connected in series on asingle substrate 51 a, 51 b, 51 c or 51 d constitute a serial array 61a, 61 b, 61 c or 61 d. Meanwhile, bonding pads 71 a, 71 b, 71 c or 71 dmay be connected to both ends of each serial array 61 a, 61 b, 61 c or61 d.

Meanwhile, the two serial arrays 61 a and 61 b are connected in seriesto each other to form an array group 81 a, and the other two serialarrays 61 c and 61 d are connected in series to each other to form anarray group 81 b. The serial arrays are connected in series throughfirst connector means 73 a and 73 b.

The first connector means 73 a may be a bonding wire for directlyconnecting the bonding pads 71 a and 71 b. Further, the first connectormeans 73 b may be a bonding wire for directly connecting the bondingpads 71 c and 71 d. However, the first connector means 73 a and 73 b arenot limited to bonding wires for directly connecting the bonding pads,but may be various connector means including lead terminals of apackage, circuit patterns of a printed circuit board, or the like.

Meanwhile, the array groups 81 a and 81 b are connected in reverseparallel to each other to operate. That is, an anode of the array group81 a and a cathode of the array group 81 b are commonly connected to aterminal, and a cathode of the array group 81 a and an anode of thearray group 81 b are commonly connected to a terminal. For example, asshown in this figure, the bonding pads 71 a and 71 b at both the ends ofthe array group 81 a and the bonding pads 71 c and 71 d at both the endsof the array group 81 b may be respectively connected to the commonterminals such that the array groups 81 a and 81 b are connected inreverse parallel to each other. Accordingly, an AC power source isconnected to the terminals to thereby drive the four single chips 50 a,50 b, 50 c and 50 d, and the array groups 81 a and 81 b alternatelyoperate depending on a change in phase of the AC power source.

Meanwhile, a second connector means 75 may electrically connect thearray groups 81 a and 81 b to each other. Although the second connectormeans 75 may connect the bonding pad 71 a between the arrays 61 a and 61b and the bonding pad 71 d between the arrays 61 c and 61 d to eachother as shown in this figure, the present invention is not limitedthereto. That is, the second connector means 75 may connect the bondingpads 71 a and 71 c and/or the bonding pads 71 b and 71 d. The secondconnector means 75 may also connect the bonding pads 71 b and 71 d toeach other. Further, the second connector means 75 may directly connectto the first connector means 73 a and 73 b. The first connector meansare electrically connected to each other by the connection of the secondconnector means 75.

Like the first connector means 73 a and 73 b, the second connector means75 may be a bonding wire for directly connecting bonding pads, or aconnector means including a lead electrode or a conductive pattern of aprinted circuit board.

If voltage is applied under an AC power source, forward voltage isapplied to any one of the array groups 81 a and 81 b, and reversevoltage is applied to the other array group. In the array group to whichthe reverse voltage is applied, uniform voltage is not applied to lightemitting cells, and overvoltage may be partially applied to specificlight emitting cells. The second connector means 75 controls thepotential between the serial arrays in the array group, to which thereverse voltage is applied, using the potential between the serialarrays in the array group, to which the forward voltage is applied.Accordingly, overvoltage can be prevented from being applied to aspecific array in the array group to which the reverse voltage isapplied, thereby protecting the light emitting cells.

In the meantime, although all the light emitting cells 58 a, 58 b, 58 cand 58 d in the single chips 50 a, 50 b, 50 c and 50 d may be formed toemit light with the same light emitting wavelength, the presentinvention is not limited thereto but may be formed to emit lights withdifferent wavelengths. Thus, according to the embodiments of the presentinvention, there may be provided an AC light emitting device foremitting light with various wavelengths as well as an AC light emittingdevice for emitting light with a single wavelength.

Meanwhile, although it has been described in this embodiment that thearrays in the two single chips are connected in series to each other,thereby forming the array group, the array group may have two or moreserial arrays formed therein. FIG. 5 is a schematic view of an AC lightemitting device according to another embodiment of the presentinvention, in which an AC light emitting device comprises array groupseach of which has three or more serial arrays.

Referring to FIG. 5, the AC light emitting device according to thisembodiment is the same as the light emitting device except the number ofsingle chips. That is, in this embodiment, three of the single chips areconnected in series to form an array group, and the other three singlechips are connected in series to form another array group. These arraygroups are connected in reverse parallel to each other.

Meanwhile, second connector means 95 a and 95 b are electrically connectthe first connector means corresponding to each other provided in thearray groups as shown in this figure. Accordingly, overvoltage can beprevented from being applied to a specific array in an array group towhich reverse voltage is applied, thereby protecting light emittingcells.

FIG. 6 is a sectional view illustrating a package on which the singlechip 50 used in the embodiments of the present invention is mounted.Here, a top-view light emitting device having a recessed package body 20will be described as an example. However, the present invention is notbe limited thereto, but may be applied to all types of packages.

Referring to FIG. 6, the single chip 50 of FIG. 1 is mounted on amounting region of the package body 20. The single chip 50 iselectrically connected to lead electrodes 30 of the package throughbonding wires 35. Meanwhile, the single chip 50 is encapsulated by amolding member 40 such as epoxy or silicone. The molding resin 40 maycontain a phosphor.

According to this embodiment, there is provided a package with a singlechip 50 mounted thereon. Such packages are connected in series, therebyforming array groups each of which has the single chips 50 connected inseries. Further, such array groups are connected in reverse parallel,thereby providing an AC light emitting device of the present invention,which can be driven under an AC power source.

Meanwhile, a plurality of the single chips 50 may be mounted in a singlepackage. For example, two or more single chips 50 are mounted in apackage and connected in series, and such two packages are connected inreverse parallel, thereby providing the AC light emitting device of thepresent invention.

The packages may be connected in series, parallel or reverse parallelusing lead terminals. The packages may also be connected using aconductive pattern on a printed circuit board on which the packages aremounted.

Meanwhile, at least four single chips may be mounted in one package. Asdescribed in FIGS. 4 and 5, the single chips constitute at least twoarray groups using bonding wires or the like, and the array groups areconnected in reverse parallel, thereby providing an AC light emittingdevice using a single package.

Although an AC light emitting device using a single chip having oneserial array on a substrate has been described in the foregoing, an AClight emitting device may be configured using single chips having serialarrays connected in reverse parallel on a substrate. FIG. 7 is aschematic view illustrating an AC light emitting device using a singlechip 100 having serial arrays connected in reverse parallel on a singlesubstrate.

Referring to FIG. 7, two serial arrays 61 a and 61 c, each of which haslight emitting cells 58 connected in series, are disposed on a substrate51. The serial arrays 61 a and 61 c are connected in reverse parallel toeach other between bonding pads 71 a and 71 b.

Such single chips 100 are connected in series by first connector means,thereby forming at least two array groups. The first connector means maybe bonding wires for directly connecting the bonding pads. That is, thearray groups may be formed by mounting the single chips in a package andthen connecting them through bonding wires. As described above, thearray groups may be formed by mounting the respective single chips 100in packages and then connecting the packages in series to one another.Various serial array groups may also be formed using single chips andpackages.

Meanwhile, the light emitting cells 58 corresponding to each otherprovided in the arrays 61 a and 61 c formed on the same substrate areelectrically connected by second connector means 105. The secondconnector means 105 prevent overvoltage from being applied to the lightemitting cells in the array to which reverse voltage is applied. Thesecond connector means 105 may be a first conductive-type lowersemiconductor layer which the adjacent light emitting cells 58 share.Alternatively, the second connector means 105 may be wires formed on thesubstrate to connect the adjacent light emitting cells.

According to this embodiment, single chips 100 having serial arrays oflight emitting cells connected in reverse parallel are directlyconnected to form array groups, so that the number of light emittingcells in a serial array on a single substrate can be reduced.

The invention claimed is:
 1. An alternating current (AC) light emittingdevice, comprising: at least two substrates; a first serial array and asecond serial array positioned on each of the substrates, the firstserial array and the second serial array being connected in reverseparallel to each other, each of the first serial array and the secondserial array comprising a plurality of light emitting cells connected inseries, the first serial array and the second serial array configured tobe driven by an AC voltage; first wires for electrically connectingserial arrays to each other on the respective different substrates,thereby forming at least two array groups; and second wires respectivelyformed on the substrates to electrically connect the light emittingcells corresponding to each other provided in the first serial array andthe second serial array.
 2. The AC light emitting device as claimed inclaim 1, wherein the first wires comprise bonding wires for connectingthe serial arrays on the respective different substrates.
 3. The AClight emitting device as claimed in claim 2, wherein the first wirescomprise lead electrodes in a package.
 4. The AC light emitting deviceas claimed in claim 1, further comprising: a first bonding pad and asecond bonding pad positioned on each of the substrates, wherein thefirst serial array and the second serial array are connected in reverseparallel to each other between the first and second bonding pads.
 5. TheAC light emitting device as claimed in claim 4, wherein a first wireamong the first wires electrically connects the first bonding pad of oneof the at least two substrates to the second bonding pad of the other ofthe at least two substrates.